Solid-state image sensing device and camera system using the same

ABSTRACT

A solid-state image sensing device includes a plurality of pixels. Each pixel has a photodiode, a first transistor, and a second transistor. The photodiode is constituted by a first-conductivity-type semiconductor region and a second-conductivity-type semiconductor region. The first and second conductivity types are opposite to each other. The first transistor has a first-conductivity-type drain region formed in the second-conductivity-type semiconductor region to transfer signal charge to the drain region. The second transistor has a source region and a drain region which are formed in the second-conductivity-type semiconductor region and which have the first conductivity type. At least one second-conductivity-type potential barrier is provided under the drain region of the first transistor and the source region and/or the drain region of the second transistor.

This is a divisional of U.S. patent application Ser. No. 11/316,868,filed Dec. 27, 2005, now pending, which is a divisional of U.S. patentapplication Ser. No. 10/944,941, filed Sep. 21, 2004, now abandoned,which is a continuation of U.S. patent application Ser. No. 10/463,604,filed Jun. 18, 2003, now U.S. Pat. No. 6,885,047, issued on Apr. 26,2005.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a solid-state image sensing device anda camera system using the same.

2. Description of the Related Art

In recent years, the demand for solid-state image sensing devices hasbeen rapidly increasing for use in image capturing apparatuses, such as,mainly, digital still cameras and video camcorders. As such solid-stateimage sensing devices, CCDs (Charge Coupled Devices) or MOS solid-stateimage sensing devices have been used. The former, as compared to thelatter, are widely used as high-definition image sensing devices, due totheir high sensitivity and low noise, but, on the other hand, have somedisadvantages. Specifically, the power consumption and the drive voltageare high and the cost is high since a general semiconductormanufacturing process cannot be used. Additionally, it is difficult tointegrate peripheral circuits, such as a drive circuit.

For these reasons, much effort has been devoted to the development of anamplifying-type MOS solid-state image sensing devices. In anamplifying-type solid-state image sensing devices, signal charge that isstored in a photodiode is introduced into the control electrode of anamplifying transistor, provided in each pixel, is amplified by theamplifying transistor, and the resulting output is output from the mainelectrode thereof in accordance with the amount of signal charge. Inparticular, for amplifying-type solid-state image sensing devices,efforts have been directed to the development of CMOS sensors using MOStransistors as amplifying transistors. The demand for portabletelephones is projected to increase, and it is expected that MOSsolid-state image sensing devices, which can overcome theabove-described disadvantages of CCDs, will be applied to portableapparatuses.

FIG. 13 is a circuit diagram of a typical example of a CMOS sensor pixelfor use in a solid-state image sensing device.

In FIG. 13, reference numeral 30 represents a unit pixel, 1 is aphotodiode for storing signal charge generated from incident light, 6 isa amplifying MOS transistor for outputting an amplified signal inaccordance with the amount of signal charge, and 3 is a floatingdiffusion (hereinafter may be referred to as “FD”) region which receivesa signal charge and connects the signal charge to the gate electrode ofthe amplifying MOS transistor 6. Reference numeral 2 represents atransfer MOS transistor for transferring signal charge stored in thephotodiode 1 to the FD region 3, 4 is a reset MOS transistor forresetting the FD region 3, and 5 is a selection MOS transistor forselecting an output pixel. Reference numeral 9 a is a control line forapplying a pulse to the gate of the transfer MOS transistor 2 to controlthe charge-transfer operation, 9 b is a control line for applying apulse to the gate of the reset MOS transistor 4 to control the resetoperation, and 9 c is a control line for applying a pulse to the gate ofthe selection MOS transistor 5 to control the selection operation.Reference numeral 10 a is a power-supply wire which is connected to thedrain of the amplifying MOS transistor 6 and the drain of the reset MOStransistor 4 to provide a power-supply potential thereto. Referencenumeral 10 b is an output line for outputting an amplified signal of aselected pixel, 8 is a constant-current MOS transistor that operates asa constant current source and that forms a source follower in conductionwith the amplifying MOS transistor 6, and 10 c is a wire for supplying apotential to the gate electrode of the MOS transistor 8 so as to operatethereof at constant current.

An arrangement of a plurality of the above-described pixels 30 in atwo-dimensional matrix provides a pixel area for a two-dimensionalsolid-state image sensing device. In the matrix configuration, theoutput line 10 b is used as a common line for pixels in the same columnand the control lines 9 a, 9 b, and 9 c are each used as a common linefor pixels in the corresponding row. Only pixels in a row that isselected by the control line 9 c output signals to the correspondingoutput line 10 b.

FIG. 14 is a circuit diagram of another pixel for a conventionalsolid-state image sensing device. In FIG. 14, reference numeral 1represents a photodiode, 2 is a transfer MOS transistor for transferringcharge of the photodiode 1, and 3 is a floating diffusion region fortemporarily storing the transferred charge. Reference numeral 4 is areset MOS transistor for resetting the floating diffusion region 3 andthe photodiode 1, 5 is a selection MOS transistor for selecting one rowin the array, and 6 is a source-follower MOS transistor. Thissource-follower MOS transistor 6 converts charge in the floatingdiffusion region 3 into a voltage and amplifies the voltage using asource-follower amplifier. Reference numeral 7 represents a read line,which is used as a common line in the same column, for reading a pixelvoltage signal, and reference numeral 8 represents a constant currentsource for providing constant current to the read line 7.

The operation of this conventional solid-state image sensing device willbe briefly described below. The photodiode 1 converts incident lightinto charge, and the transfer MOS transistor 2 causes the charge to bestored in the floating diffusion region 3. The potential of the floatingdiffusion region 3 and the photodiode 1 is reset to a constant potentialin advance by opening the reset MOS transistor 4 and the transfer MOStransistor 2. Thus, the potential of the floating diffusion region 3varies in accordance with charge generated from incident light.

The potential of the floating diffusion region 3 is amplified by thesource-follower MOS transistor 6 and is output to the read line 7. Whenthe selection MOS transistor 5 is open, that pixel is selected. Anoutput circuit (not shown) detects optical signal components bydetermining the difference between the potential of the floatingdiffusion region 3 after optical signals are stored and the resetpotential of the floating diffusion region 3.

FIG. 15 is a schematic sectional view of the solid-state image sensingdevice shown in FIG. 13. This schematic sectional view includes portionscorresponding to the photodiode, the transfer MOS transistor, and the FDregion. In this figure, reference numeral 11 represents an n-typesemiconductor substrate, 12 is a p well, and 15 is an n-typesemiconductor region formed in the p well 12. The p well 12 and then-type semiconductor region 15 constitute a photodiode. Signal chargegenerated from incident light is stored in the n-type semiconductorregion 15. Reference numeral 14 is a gate electrode of the transfer MOStransistor 2 shown in FIG. 13. Reference numeral 18 represents an FDregion, which is an n-type semiconductor region formed in the p well 12and also serves as the drain region of the transfer MOS transistor 2.The source region of the transfer MOS transistor corresponds to then-type semiconductor region 15. Reference numeral 20 represents a wirethat is connected to the FD region 18 and also to the gate electrode ofan amplifying MOS transistor (not shown). Reference numeral 17 is anelement-isolating insulating film, which is called a “LOCOS” oxide film.Reference numeral 29 is a p+ channel stopper, which is formed under theelement-isolating insulating film 17 and has a doping concentrationhigher than the p well 12.

FIG. 16 is a schematic sectional view of the solid-state image sensingdevice shown in FIG. 14. This sectional view shows a combination ofportions corresponding to the photodiode 1 and the transfer MOS transfer2 shown in FIG. 14. Reference numeral 11 represents an n-type siliconsubstrate, 12 is a p well, 13 a is a gate oxide film of the transfer MOStransistor 2, 13 b is a thin oxide film provided on a light-receivingportion, 14 is the gate electrode of the transfer MOS transistor 2, and15 is an n-type cathode of the photodiode 1. Reference numeral 16represents a p-type surface region for providing a photodiode-buriedstructure, and 17 is a LOCOS oxide film for element isolation. Referencenumeral 18 is a heavily-doped n-type region that forms a floatingdiffusion region and also acts as the drain region of the transfer MOStransistor 2. Reference numeral 19 is a silicon oxide film for providinginsulation between the gate electrode and a first metal layer 21.Reference numeral 20 is a contact plug, 22 is an interlayer insultingfilm for providing insulation between the first metal layer 21 and asecond metal layer 23, 24 is an interlayer insulating film for providinginsulation between the second metal layer 23 and a third metal layer 25,and 26 is a passivation film. For a color photoelectrical conversiondevice, a color filter layer (not shown) is formed at the upper layer ofthe passivation film 26 and a micro-lens (not shown) is further formedthereon to improve the sensitivity. Incident light through the surfaceenters the photodiode through an aperture where the third metal layer 25is not provided. The light is absorbed by the n-type cathode 15 of thephotodiode or the p well layer 12, so that electron-hole pairs areproduced. Of these pairs, electrons are stored in an n-type cathoderegion.

U.S. Pat. No. 6,403,998 discloses a solid-state image sensor in which ap-type buried layer is formed at a predetermined distance from an n-typesubstrate and a photoelectric conversion section is formed thereabove.In addition, U.S. Pat. No. 6,504,193 discloses a solid-state imagedevice in which one end of a photodiode is formed to extend to aposition under a readout gate and a punch-through stopper region isformed under a signal detection portion, which corresponds to the drainregion, to be in self-alignment with the gate electrode.

With the conventional structures shown in FIGS. 15 and 16, however, partof signal charge generated below the photodiode is not absorbed by thephotodiode, and is, in turn, absorbed by the FD region 18 and the sourceand drain regions of the transistor within the pixel. As a result, thesensitivity decreases.

Additionally, although various improvements have been made to CMOSsolid-state image sensing devices, there is still a problem in that thesensitivity is low, particularly, in a device having a small pixel size.The present invention provides a CMOS solid-state image sensing devicethat has a novel structure and that can provide high sensitivity evenfor micro pixels.

A description is now given to a reason why the sensitivity in theconventional structure shown in FIG. 16 is low. Referring to FIG. 16,electrons that are generated from a light ray 27 entering the apertureare successfully stored in the n-type cathode region and serve as aneffective signal charge. However, for example, as in the case of a lightray 28, electrons that are generated at a position somewhat away fromthe photodiode may be captured, not by the n-type cathode region, but bythe n+ type floating diffusion region 18 where the potential is lower.In addition, even immediately under the photodiode, as a result ofrepeated diffusion and drift of electrons, the electrons are absorbed bya low potential region other than the photodiode with a certainprobability and thus do not contribute as photoelectric conversionsignals. When the n-type cathode 15 is formed at a position deeperrelative to the silicon surface, such an arrangement facilitates thephotodiode to collect the electrons. However, since the n-type cathoderegion is formed in the p well region 12 by ion implantation, the dopingconcentration cannot be reduced so significantly. This conventionalstructure also has a problem in that the n-type cathode 15 cannot beformed with a high doping concentration at a considerably deep position,due to the limitation of depletion behavior of the n-type cathode 15.

Thus, the volume of the n-type cathode, which provides the photodiode,is limited. Consequently, a sufficient ability of collecting electronsgenerated from incident light cannot be achieved, resulting in lowsensitivity.

Meanwhile, the conventional structure disclosed in U.S. Pat. No.6,403,998 also cannot prevent electrons that are generated at a deepposition in response to incident light from being absorbed by thefloating diffusion region 18 or the like, since no potential barrier isprovided under the signal readout gate. Thus, this structure also has aproblem in that the sensitivity decreases. Also, the conventionalstructure disclosed in U.S. Pat. No. 6,504,193 cannot prevent some ofelectrons that are generated from incident light from being absorbed bythe source and drain of another transistor in the pixel or from beingabsorbed by adjacent pixels, since the punch-through stopper region isformed only under a signal detection portion. Thus, this structure alsohas a problem in that the sensitivity decreases as well.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a solid-state imagesensing device that has high sensitivity, particularly, for micropixels, and another object of the present invention is to provide acamera system that are low in power consumption and in drive voltage andlow in cost.

To achieve the foregoing objects, according to a first aspect of thepresent invention, there is provided a solid-state image sensing device.The solid-state image sensing device has a plurality of pixels. Eachpixel includes a photodiode that is constituted by a semiconductorregion having a first conductivity type and a semiconductor regionhaving a second conductivity type to generate signal charge. The firstconductivity type and the second conductivity type are opposite to eachother. Each pixel further includes a first transistor that has a drainregion, which has the first conductivity type formed in thesecond-conductivity-type semiconductor region to transfer the signalcharge to the drain region, and a second transistor that has a sourceregion and a drain region, which are formed in thesecond-conductivity-type semiconductor region and which have the firstconductivity type. At least one potential barrier having the secondconductivity type is provided under the drain region of the firsttransistor and the source region and/or the drain region of the secondtransistor. With arrangement, since the at least one potential barrieris provided under an FD region and the source electrode and/or the drainelectrode of each transistor, thereby increasing the sensitivity. Thisis because signal charge that is generated below the potential barrieris not absorbed by the FD region and the source region and/or the drainregion, under which the potential barriers are provided, of eachtransistor.

According to a second aspect of the present invention, there is provideda solid-state image sensing device. The solid-state image sensing has atleast one pixel. Each pixel includes a photodiode that is constituted bya semiconductor region having a first conductivity type and asemiconductor region having a second conductivity type. The firstconductivity type and the second conductivity type are opposite to eachother. Each pixel further includes a transistor that has a source regionand a drain region which are provided in the first-conductivity-typesemiconductor region and which have the first conductivity type. Apotential barrier having the second conductivity type is provided underthe gate electrode of the transistor. With this arrangement, thepotential barrier that is provided under the gate electrode of an n-typetransistor, which constitutes the pixel, prevents short-circuitingbetween the source and drain. The potential barrier also serves to allowthe n-type transistor, provided even in an n-type semiconductor region,to operate.

According to a third aspect of the present invention, there is provideda solid-state image sensing device. The solid-state image sensing deviceincludes a substrate having a first conductivity type, a layer having asecond conductivity type, and a layer having the first conductivitytype. The second-conductivity-type layer and the first-conductivity-typelayer form a photodiode. At least one potential barrier, which is formedby a region having the second conductivity type, is provided around aregion where the photodiode is formed.

The second-conductivity-type region may be provided in thefirst-conductivity-type layer and the second-conductivity-layer may be aburied layer. The second-conductivity-type region may extend to thesecond-conductivity-type buried layer in the depth direction. With thisarrangement, when an n-type layer for the photodiode is surrounded bythe buried layer having the opposite conductivity type and the layerhaving the same conductivity type as that of the buried layer, potentialbarriers are provided against electrons. In addition, this arrangementallows the photodiode to have a low doping concentration and to have alarge volume, thereby increasing a collecting ability of opticalsignals.

The potential barrier may be constituted by a plurality of potentialbarriers that is arranged at a plurality of layers in the depthdirection of the first-conductivity-type layer. With this arrangement,in a structure having a deep n-type layer for the photodiode, that is,in a structure having sensitivity even at a longer wavelength, providinga plurality of p-type layers allows for formation of effective potentialbarriers.

The uppermost layer of the plurality of layers may control a chargetransfer path from the photodiode to a transfer transistor. Thisarrangement can achieve a structure in which the transfer transistor cansecure transfer of signals from the photodiode.

A portion in at least the vicinity of the opposite-conductivity-typeburied layer in the first-conductivity-type layer may have a lowerdoping concentration than the surrounding opposite-conductivity-typelayer. With this arrangement, when the photodiode is reverse biased atthe time of storing charge, a depletion layer effectively extends towardthe n-type layer to facilitate complete depletion. Since the well layerand the buried layer are placed at substantially the same position inthe horizontal plane direction, less photomask processing is required.Thus, this arrangement can provide a higher isolation effect.

The first-conductivity-type layer may have a region having a dopingconcentration higher than the other regions, in a portion adjacent tothe semiconductor surface. With this arrangement, electron potentials atthe semiconductor surface become lower than in the other portions, andthus electrons gather in the vicinity of the surface at the time ofstoring charge. Consequently, electrons are less likely to remain at thetime of transferring charge.

The first-conductivity-type layer may be completely depleted duringcharge transfer. This arrangement can provide a “complete-transfer”photodiode, which allows for the determination of the concentration inthe depth direction so that the photodiode that is surrounded by theopposite-type-conductivity layer is depleted due to reverse biasing andwhich has no reset noise.

According to a fourth aspect of the present invention, there is provideda method for manufacturing the solid-state image sensing device of thethird aspect of the present invention. In this manufacturing method,after the first-conductivity-type layer is formed on theopposite-conductivity-type buried layer, the opposite-conductivity-typelayer is formed by ion implantation. This manufacturing method allowsfor the manufacture of a solid-state image sensing device that can dealwith various spectral characteristics, by changing the thickness of theepitaxial layer.

According to a fifth aspect of the present invention, there is provideda method for manufacturing the solid-state image sensing device of thethird aspect of the present invention. In this manufacturing method, theopposite-conductivity-type buried layer is formed in such a manner that,after ions are implanted into the first-conductivity-type semiconductorsubstrate, ions are implanted into the opposite-conductive-type buriedlayer. This manufacturing method allows for formation of a uniformburied layer within a surface in the depth direction with highreproducibility by ion implantation, when there is no need to have asignificantly high spectral sensitivity at a longer wavelength.

In addition, an isolation region may be provided between adjacent pixelsby STI (Shallow Trench Isolation). Since STI is used, even for minutepixels, it is possible to provide an image sensing device which has lesscharge leakage between adjacent pixels, which has low crosstalk, andwhich has high sensitivity.

The entire contact surface between an oxide film, which is formed by theSTI and which isolates pixels, and silicon may be covered by thefirst-conductivity-type semiconductor layer. With this arrangement, adepletion layer is not formed at the interface between the oxide filmand the silicon during STI, which can provide an image sensing devicehaving less noise when it is dark.

An isolation region may be provided between adjacent pixels by deeptrench isolation. With this arrangement, for minute pixels, isolationsbetween the pixels can further be enhanced, which can provide an imagesensing device having low cross talk and having high sensitivity.

The entire contact surface between an oxide film, which is formed by thedeep trench isolation and which isolates pixels, and silicon may becovered by the first-conductivity-type semiconductor layer. With thisarrangement, a depletion layer is not formed at the interface betweenthe oxide film and the silicon during deep trench isolation, which canprovide an image sensing device having less noise when it is dark.

Further objects, features and advantages of the present invention willbecome apparent from the following description of the preferredembodiments with reference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic sectional view of a solid-state image sensingdevice according to a first embodiment of the present invention.

FIG. 2 is a schematic sectional view of a solid-state image sensingdevice according to a second embodiment of the present invention.

FIG. 3 is a schematic sectional view of a solid-state image sensingdevice according to a third embodiment of the present invention.

FIG. 4 is a schematic sectional view of a solid-state image sensingdevice according to a fourth embodiment of the present invention.

FIG. 5 is a schematic sectional view of a solid-state image sensingdevice according to a fifth embodiment of the present invention.

FIG. 6 is a schematic sectional view of a solid-state image sensingdevice according to a sixth embodiment of the present invention.

FIG. 7 is a schematic sectional view of a solid-state image sensingdevice according to a seventh embodiment of the present invention.

FIG. 8 is a plan view showing one example of a pixel of a solid-stateimage sensing device according to the present invention.

FIG. 9 is a schematic sectional view of a solid-state image sensingdevice according to an eighth embodiment of the present invention.

FIG. 10 is a schematic sectional view of a solid-state image sensingdevice according to a ninth embodiment of the present invention.

FIG. 11 is a circuit diagram of part of the solid-state image sensingdevice according to the present invention.

FIG. 12 is a block diagram of a camera system using the solid-statesensing device according to the present invention.

FIG. 13 is a circuit diagram of a typical example of a CMOS sensor pixelfor use in a solid-state image sensing device.

FIG. 14 is a circuit diagram of a pixel of a conventional solid-stateimage sensing device.

FIG. 15 is a schematic sectional view of the solid-state image sensingdevice shown in FIG. 13.

FIG. 16 is a schematic sectional view of the solid-state image sensingdevice shown in FIG. 14.

DESCRIPTION OF THE PREFERRED EMBODIMENTS First Embodiment

FIG. 1 is a schematic sectional view of a solid-state image sensingdevice according to a first embodiment of the present invention.

In FIG. 1, reference numeral 101 represents a semiconductor substratehaving a first-conductivity-type (n-type in this case by way ofexample), 102 is a p well which is a second-conductivity-typesemiconductor region, and 103 is an n-type semiconductor region which isformed in the p well 102 and which is a first-conductivity-typesemiconductor region. The p well 102 and the n-type semiconductor region103 constitute a photodiode. Signal charge that is generated fromincident light is stored in the n-type semiconductor region 103.Reference numeral 111 represents a drain region of a transfertransistor, which is a first transistor, for transferring signal chargegenerated by the photodiode. The drain region has the first conductivitytype and serves as an FD region and an n-type semiconductor regionformed in the p well 102. Reference numeral 109 represents a gateelectrode of the transfer transistor. The n-type semiconductor regions103 and 111 serve as the source region and the drain region of thetransfer transistor, respectively. Reference numeral 113 is a wire thatis connected to the n-type semiconductor region 111 and also to a gateelectrode 130 of an amplifying transistor, which is a second transistor.The amplifying transistor is constructed such that the gate electrode130 is provided between a source region 131 and a drain region 132.Reference numeral 133 is a vertical signal line that is connected to thesource region 131. The drain region 132 serves not only as the drainelectrode of the amplifying transistor but also as the source region ofa selection transistor, which is another second transistor. Referencenumeral 134 represents a drain electrode of the selection transistor,and a gate electrode 135 is formed between the source electrode 132 anddrain electrode 134. Reference numeral 136 is a drain line that isconnected to the drain electrode 134 of the selection transistor.Reference numeral 107 represents element-isolating insulating films thatare formed with thick oxide films. Reference numeral 106 represents p+channel stoppers that are formed under the element-isolating insulatingfilms 107 and that have a higher doping concentration than the p well102. Reference numeral 105 represents potential barriers. The potentialbarriers 105 are formed with p-type semiconductor regions, which havethe same conductivity type as the p well 102.

Herein, it should be noted that a transfer transistor is referred to asa “first transistor” and transistors, other than the transfertransistor, which are formed in a pixel are referred to as “secondtransistors”.

Signal charge stored in the n-type semiconductor region 103 istransferred to the FD region 111 during a transfer operation. Theconcentration of n-type dopants in the n-type semiconductor region 103is set such that the n-type semiconductor region 103 is depletedimmediately after a transfer.

In the solid-state image sensing device of this embodiment, one pixel isconstituted by the photodiode, transfer transistor, amplifyingtransistor, and selection transistor. The configuration of one pixel,however, is not limited to this particular embodiment. For example, onepixel may be constituted by a transfer transistor and amplifyingtransistor. Alternatively, one pixel may be constituted by a photodiode,transfer transistor, amplifying transistor, selection transistor, andreset transistor.

The Potential barriers 105 in the present invention are characterized inthat they are provided under the drain region of the first transistorand the source regions and/or the drain regions of the secondtransistors. When a plurality of second transistors is formed, at leastone potential barrier 105 is provided so as to correspond to at leastone source region and/or drain region thereof.

The potential barriers 105 have a p-type impurity at a concentrationhigher than the p well 102. While the channel stoppers 106 and thepotential barriers 105 are semiconductor regions having the same p+type, the doping concentrations thereof may, of course, be differentfrom each other. The potential barriers 105 are provided by implanting,for example, boron or gallium into the p well 102 using ionimplantation.

As shown in FIG. 1, the potential barriers 105 in the present inventionare formed under the FD region 111 and the source and drain regions 131,132, and 134 of the amplifying transistor and the selection transistor.

As in the present invention, providing the potential barriers 105 underan FD region and the source electrode and/or the drain electrode of eachtransistor can improve the sensitivity. This is because signal chargethat is generated below the potential barriers 105 is not absorbed bythe FD region 111 and the source region and/or the drain region, underwhich the potential barriers 105 are provided, of each transistor.

In addition, this arrangement is preferable because providing morepotential barriers 105 under the source and drain regions of the secondtransistors provides a structure in which signal charge is less likelyto be absorbed by regions other than the photodiode.

The potential barriers 105 may also be provided under theelement-isolating insulating films 107. Providing the potential barriers105 under the element-isolating insulating films 107 can provide astructure in which signal charge is less likely to be absorbed by aphotodiode or transistor within an adjacent pixel, thereby preventingsignal charges between the adjacent pixels from being mixed.

Second Embodiment

FIG. 2 is a schematic sectional view of a solid-state image sensingdevice according to a second embodiment of the present invention.

More specifically, FIG. 2 is a schematic sectional view of a solid-stateimage sensing device having a photodiode, a transfer transistor and areset transistor for resetting an FD electrode 211. Thus, thissolid-state image sensing device has a reset transistor, serving as asecond transistor, for resetting the FD electrode 211.

In FIG. 2, reference numeral 223 represents a gate electrode of thereset transistor for resetting the FD electrode 211 and referencenumeral 224 represents a drain region of the reset transistor and isconnected to a power-supply line 226.

This embodiment is different from the first embodiment described abovein that a potential barrier 205 is also provided under the gateelectrode 223 of the second transistor. Thus, the potential barrier 205reduces the amount of signal charge that is absorbed by n-typesemiconductor regions other than the photodiode, thereby enhancing thesensitivity.

While the second transistor in this embodiment has been described inconjunction with the reset transistor by way of example, it may be anamplifying transistor or selection transistor as in the firstembodiment. In addition, a plurality of second transistors may beprovided.

For example, the potential barriers 205 of the present invention may beprovided in a solid-state image sensing device in which each pixel isconstituted by a photodiode, transfer transistor, reset transistor,amplifying transistor, and selection transistor.

Providing the potential barriers 205 under the gate electrodes, sourceregions, and drain regions of all the transistors provided within apixel makes it more difficult for signal charge to be absorbed byregions other than the photodiode, thereby improving the sensitivity.

In addition, providing the potential barriers 205 under theelement-isolating insulating regions 207 can provide a solid-state imagesensing device in which the potential barriers 205 are provided inregions deeper than the source and drain regions of the transistors soas to surround the photodiode.

As described above, providing the potential barriers 205 around thephotodiode makes it difficult for signal charge, generated by thephotodiode, to be absorbed by the source or drain region of the adjacenttransistor, thereby enhancing the sensitivity.

The potential barriers 205 that are provided so as to surround thephotodiode may have at least one opening.

With a structure in which the potential barriers 205 without an openingtherein are provided around the photodiode, when signal charge overflowsfrom the photodiode, blooming is prone to occur since the signal chargethat overflows therefrom is not easily absorbed by the surroundingn-type semiconductor region. Accordingly, it is preferable that anopening having no potential barrier 205 therein be provided in at leastone portion around the photodiode to absorb signal charge that overflowstherefrom so that blooming can be suppressed.

Third Embodiment

FIG. 3 is a schematic sectional view of a solid-state image sensingdevice according to a third embodiment of the present invention.

Specifically, FIG. 3 is a schematic sectional view of a solid-stateimage sensing device having a photodiode, a transfer transistor, and areset transistor for resetting an FD electrode 311.

Referring to FIG. 3, an n well or n-type semiconductor region 303 isprovided as a first-conductivity-type semiconductor region at a layerabove an n-type semiconductor substrate 301. A p-type semiconductorregion 302 is provided as a second-conductivity-type semiconductorregion. The p-type semiconductor region 302 and the n-type semiconductorregion 303 constitute a photodiode. A first-conductivity-typesignal-charge storing region 312 collects and stores signal chargegenerated by the photodiode and has a doping concentration higher thanthe n-type semiconductor region 303.

The difference between the configurations shown in FIGS. 2 and 3 is asfollows. In the configuration shown FIG. 2, in the p well 202, thetransistors having the n-type source and drain regions, which type beingopposite to that of the p well, are formed, and also the potentialbarriers 205 have the same conductivity type as the p well 202. Incontrast, in this embodiment shown in FIG. 3, in the n-typesemiconductor region 303 which is a first-conductivity-typesemiconductor region, transistors having the source and drain regions ofthe same conductivity type as that of the n-type semiconductor region303 are formed. Further, p-type potential barriers 305 having anopposite conductivity type to that of the n-type semiconductor region303 are provided.

In this embodiment, as transistors that constitute a pixel inconjunction with the photodiode, a transfer transistor for transferringsignal charge generated by the photodiode and a reset transistor forresetting the FD electrode 311 are illustrated. The transistors,however, are not limited to this particular configuration. For example,transistors provided in the pixel may be any one or a combination of atransfer transistor, reset transistor, amplifying transistor, selectiontransistor, and the like.

In this embodiment, the potential barriers 305 that are provided underthe gate electrodes of the n-type transistors, which constitute thepixel, prevent short-circuiting between the sources and drains. Thepotential barriers 305 also serve to allow the n-type transistors,provided even in the n-type semiconductor region 303, to operate.

The potential barriers 305 that are placed under the gate electrodes maybe sized so as to correspond to the gate regions or may be formed underparts of the gate electrodes.

Thus, in this embodiment, the potential barrier 305 that is placed underthe gate electrode 309 of the transfer transistor is positioned underpart of the gate electrode 309, and the potential barrier 305 that issized to correspond to the gate electrode 323 is provided under the gateelectrode 323 of the reset transistor.

Herein, the potential barriers provided under the source and drainregions may also be positioned under parts of the source and drainregions.

In a pixel structure as shown in FIG. 3, since the n-type region of thephotodiode is formed deep in the light traveling direction, quantumefficiency for the signal charge is enhanced, even compared to the firstembodiment.

In the third embodiment, since the n-type transistors in the pixel areformed in the n-type semiconductor region, the threshold potential ofthe n-type transistors becomes lower than the threshold potential of theconventional n-type transistors. This makes it possible to increase theinput/output range of the amplifying transistor.

The threshold potential of the transistors in this embodiment allows afluctuation due to a back-gate effect to decrease and allows an increasein the gain compared to the conventional configurations.

The potential barriers 305 may also be provided under theelement-isolating insulating regions 307. Providing the potentialbarriers 305 under the element-isolating insulating films 307 canprovide a structure in which signal charge is less likely to be absorbedby a photodiode or transistor in an adjacent pixel, thereby preventingsignal charges between the adjacent pixels from being mixed.

Fourth Embodiment

FIG. 4 is a schematic sectional view of a solid-state image sensingdevice according to a fourth embodiment of the present invention.

This embodiment is different from the third embodiment shown in FIG. 3in that a potential barrier 405 is provided not only under the gate of atransistor but also under the source region and/or the drain region ofthe transistor provided in a pixel.

Providing the potential barrier 405 under the source region and/or thedrain region as well as under the gate can provide a structure in whichsignal charge generated below the potential barrier 405 is less likelyto be absorbed by the source region and/or the drain region of thetransistor, thereby further enhancing the sensitivity.

In this embodiment, a transfer transistor for transferring signal chargegenerated by the photodiode and a reset transistor for resetting an FDelectrode are illustrated as transistors provided in the pixel. Thetransistors, however, are not limited to this particular configuration.For example, the transistors in the pixel may be any one or acombination of a transfer transistor, reset transistor, amplifyingtransistor, selection transistor, and the like.

The potential barrier 405 may also be provided under theelement-isolating insulating film 407. Providing the potential barrier405 under each element-isolating insulating film 407 can provide astructure in which signal charge is less likely to be absorbed by aphotodiode or transistor in an adjacent pixel, thereby preventing signalcharges between the adjacent pixels from being mixed.

As described above, providing the potential barriers 405 around thephotodiode makes it difficult for signal charge, generated by thephotodiode, to be absorbed by the source or drain region of the adjacenttransistor, thereby enhancing the sensitivity compared to a case inwhich no potential barrier is provided. The potential barriers 405 thatare provided so as to surround the photodiode may have at least oneopening therein where no potential barrier is provided.

With a structure in which the potential barriers 405 without an openingtherein are provided around the entire periphery of the photodiode, whensignal charge overflows from the photodiode, blooming is prone to occursince the signal charge that overflows therefrom is not easily absorbedby the surrounding n-type semiconductor region. Accordingly, providingan opening, where the potential barrier 405 is not provided, in at leastone portion around the photodiode allows blooming to be suppressed byabsorbing signal charge that overflows from the photodiode.

As an alternative for suppressing blooming, a so-called “verticaloverflow drain” structure is available. In the structure, the dopingconcentration of a p-type semiconductor region 402 is made lower thanthe doping concentration of the potential barriers 405 to permit signalcharge that overflows from the photodiode to escape into an n-typesemiconductor substrate 401. Such a structure can eliminate the need forproviding the above-described opening, thereby enhancing the sensitivitycompared to a case in which the above-described opening is not provided.

In this embodiment, a signal-charge storing region 412 is provided, butdoes not necessarily have to be provided. A so-called “buriedphotodiode” may be employed. That is, a p-type semiconductor region maybe formed at the semiconductor interface portion of the signal-chargestoring region 412 or the n-type semiconductor region in which signalcharge is stored.

As the pixel structure in the first to fourth embodiments describedabove, a structure in which the polarities of the n-type and p-typelayers and portions are all inverted may be used.

Fifth Embodiment

A fifth embodiment will now be described with reference to FIG. 5. FIG.5 is a schematic sectional view of a solid-state image sensing deviceaccording to a fifth embodiment of the present invention.

In FIG. 5, a photodiode portion, a transfer MOS transistor portion, anda floating diffusion portion within a pixel are shown, as in theconventional example shown in FIG. 16. Reference numeral 501 is ann-type silicon substrate, 502 is a heavily-doped p-type buried layer,503 is an n-type epitaxial layer that acts as the cathode of thephotodiode, 504 a and 504 b are p-type isolation layers, and 505 a and505 b are p well layers. Reference numeral 506 a is a channel-stopp-type layer that is provided under a field oxide film 507. Referencenumeral 508 is a gate oxide film of the transfer MOS transistor, 509 isthe polysilicon gate of the transfer MOS transistor, and 510 is a p-typesurface layer for providing a buried-photodiode structure. Referencenumeral 511 is an n-type drain diffusion region for the transfer MOStransistor. The n-type drain diffusion region 511 acts as a floatingdiffusion portion that temporarily stores transferred charge. Further,reference numeral 512 represents a first interlayer insulating film, 513is a contact plug, 514 is a first-layer wiring layer, 515 is a secondinsulating film, 516 is a second-layer wiring layer, 517 is a thirdinterlayer insulating film, 518 is a third-layer wiring layer, and 519is a passivation film.

While three wiring layers are formed in this embodiment, the number ofwiring layers may be one or two to ensure optical properties dependingon the specifications of the sensor. It is to be noted that such aconfiguration is also consistent with the essence of the presentinvention. When the solid-state image sensing device of this embodimentis used as a color-image sensing device, forming a color filter layer onthe passivation film and further forming a micro-lens above the colorfilter layer can improve the photosensitivity, in the same manner as inconventional image sensing devices. Also, the p-type isolation layers504 a and 504 b and the p well layers 505 a and 505 b, together with theheavily-doped p-type buried layer 502, surround the n-type epitaxialregion 503 that acts as the cathode of the photodiode. This structureprovides electrical isolation between the photodiodes of adjacentpixels.

Of electron-hole pairs generated in the pixel from incident light rays520 and 521, electrons are securely stored as signal charge in thephotodiode by potential barriers provided by the various p-type layerssurrounding the n-type epitaxial layer 503. The p well layer 505 a isplaced almost immediately under the transfer MOS transistor, and alsocontrols a transfer path for transferring electrons that are stored inthe n-type epitaxial layer 503 to the floating diffusion portion 511through the channel of the transfer MOS transistor. Appropriatelydesigning the concentrations, depths, and horizontal directions of the pwell layers 505 a and 505 b enables the n-type epitaxial layer 503 to becompletely depleted in response to application of a read voltage to theMOS gate 509.

The p well layer 505 a, which is electrically connected with the p-typeisolation layer 504 a, acts as a well for the charge transfer MOStransistor to control the threshold voltage thereof. Another p-typelayer 505 a is also arranged under another transistor within the pixeland also acts as a well for the transistor. The heavily-doped p-typeburied layer 502 is arranged at a depth that provides spectralcharacteristics needed by the sensor. In terms of forming the potentialbarriers, a doping concentration of 1E15 (/cm³) or more is sufficient.Also, in order to reduce electrical resistance, it is desirable to formthe heavily-doped buried layer 502 with a high concentration. However,implantation with a high dosage of ions with high energy leads to anincrease in manufacturing cost. Thus, in reality, the upper limit of theconcentration depends on such a factor.

The horizontal positions of the p-type isolation layers 504 a and 504 bare determined, based on the aperture area of the photodiode and aposition at which incident light can reach. It is sufficient for thedoping concentration to be 1E15 (/cm³) or more in order to allow for theformation of the potential barriers. When the thickness of the n-typeepitaxial layer is greater than that in this embodiment, a second p-typeisolation layer may be added to provide a structure in which the p-typelayers are coupled in the depth direction.

A method for manufacturing the structure of this embodiment will now bedescribed. There are generally two types of methods for forming theheavily-doped p-type buried layer 502.

In the first method, after boron ions are implanted into the surface ofthe n-type silicon substrate 501, the n-type silicon layer 503 isepitaxially grown. The doping concentration of the n-type substrate 501is preferably in the range of 1E13 to 1E15 (/cm³). The dopingconcentration of the n-type buried layer 502 may be in the range of 1E15to 1E20 (/cm³). The doping concentration of the n-type epitaxial layer503 may be in the range of 1E14 to 1E16 (/cm³). The thickness of then-type epitaxial layer 503 is set depending on a required spectralsensitivity. The thickness thereof is preferably about 2 to 6 μm for atypical visible-light image sensing device.

In the second method, with an accelerated energy of 1 MeV to 5 MeV,boron ions are implanted through the surface of an n-type siliconsubstrate having a doping concentration of 1E14 to 1E16 (/cm³) or anepitaxially-grown n-type silicon substrate, to form the heavily-dopedp-type buried layer 502. The surface side of the heavily-doped p-typeburied layer 502 becomes the n-type epitaxial layer 503 shown in FIG. 5.Next, the p-type isolation layers 504 a and 504 b and then the p welllayers 505 a and 505 b are formed by patterning and ion implantation.The net doping concentration of the p-type isolation layers 504 a and504 b, i.e., the doping concentration thereof after being cancelled outby the doping concentration of the underlying n-type silicon, can be inthe range of 1E15 to 1E18 (/cm³).

Designing the isolation layers 504 a and 504 b to have a dopingconcentration higher than that of the n-type epitaxial layer 503 allowsthe p-type layers to act as potential barriers without being depletedeven when the pn junction of the photodiode is reverse biased. When thethickness of the epitaxial layer is about 4 μm as in this embodiment,the ion implantation range in the p-type isolation layers 504 a and 504b is, preferably, 1.5 to 3.0 μm.

In this embodiment, boron is used as the ion species and a dosage of6E11 (cm⁻²) is implanted with an energy of 1,200 KeV. Under theseconditions, the range of boron ions becomes 1.9 μm, which can provideelectrical connection between the p-type isolation layers 504 a and 504b and the heavily-doped p-type buried layer 502.

With regard to the p well layers 505 a and 505 b, the ion implantationrange is, preferably, 0.5 to 1.5 μm. In this embodiment, boron is usedas the ion species and a dosage of 1E12 (cm⁻²) is implanted with anenergy of 500 KeV. Under these conditions, the range of boron ionsbecomes 1.0 μm, which can provide electrical connection between the pwell layer 505 a and the p-type isolation layer 504 a and between the pwell layer 505 b and the p-type isolation layer 504 b.

The conditions for forming the p-type isolation layers 504 a and 504 band p well layers 505 a and 505 b vary depending on the thickness of theepitaxial layer 503. When the thickness of the epitaxial layer is 6 μmor more, a structure having p-type isolation layers at two layers ispreferable in order to electrically connect the p well layers with theheavily-doped p-type buried layer. When the thickness of the epitaxiallayer is 2 μm or less, there is no need to provide the p-type isolationlayers. The thickness of the epitaxial layer defines the spectralsensitivity at a longer wavelength of an image sensing device, and, in atypical visible light band, 4 μm is sufficient for the thicknessthereof. Thus, the structure of this embodiment is advantageously usedin an image sensing device for a visible light band.

Next, the channel stop layers 506 a and 506 b are formed by ionimplantation, and then the field oxide films 507 are formed by commonLOCOS separation or a recessed LOCOS process. After the polysiliconelectrode 509 is formed, the p-type surface layer 510 and theheavily-doped n-type layer 511 are formed on the surface of thephotodiode by ion implantation. In this manufacturing method, sinceprocesses after making the contact are analogous to those for theconventional image sensing devices, the description thereof will beomitted.

According to this embodiment, photocarriers that cannot be captured bythe conventional photodiodes can also be captured, thereby improving thesensitivity. In this embodiment, although the epitaxial layer 503 has ann-conductivity type, it may have a p-conductivity type. Thus, naturally,the present invention is also applicable to a case in which all theconductivity types of those portions and layers are inverted toconstitute a hole-storing pixel.

FIG. 8 is a plan view of one example of a pixel in the presentinvention. A p well-layer and isolation-layer region 802 is arranged asindicated by a dotted line so as to surround a photodiode 801. Atransfer-transistor gate line 803 for transferring charge is arranged atone side of the photodiode 801. A floating diffusion region 804 is alsoprovided to temporarily store transferred charge.

While an amplifying MOS transistor, a reset MOS transistor, and arow-selection MOS transistor are not shown in this plan view, theillustrated configuration is not intended to limit the arrangement ofthose devices to achieve the present invention. The main point in thisplan view is that well and isolation layers surround the photodiode toprovide isolation from adjacent pixels.

FIG. 11 is a schematic view of a circuit in which a large number ofpixel circuits of the present invention are arranged in two dimensions.Pixels 1101 each have a photodiode 1102, a transfer MOS transistor 1103,an amplifying MOS transistor 1104, a reset MOS transistor 1105, and aselection MOS transistor 1106. The gates of the selection MOStransistors 1106 in the same row are connected to the same selectionline 1107, the gates of the reset MOS transistors 1105 in the same roware connected to the same reset line 1108, and the gates of the transferMOS transistors 1103 are connected to the same transfer line 1109. Theselines 1107, 1108, and 1109 are scanned and selected by a vertical scancircuit 1110. Current sources 1112 are connected to the output lines1111 in the corresponding columns, and the potentials of the outputlines 1111 can be read by a source-follower operation.

The optical-signal-transfer MOS transistor 1113 that is selected by anoptical-signal read line 1115 causes an optical signal on thecorresponding output line 1111 to be stored in a charge storing section1118, and, similarly, a noise-signal transfer MOS transistor 1114 thatis selected by a noise-signal read line 1116 causes a noise signal to bestored in the charge storing section 1118. Optical and noise signalsthat are stored in the charge storing section 1118 are sequentiallyscanned and read by a horizontal scan circuit 1117. The difference of anoptical signal and a noise signal which are read is output by adifferential amplifier circuit (not shown).

FIG. 12 shows a circuit block of an example of a camera systemincorporating the solid-state image sensing device of the presentinvention. A shutter 1201 is placed in front of an image-taking lens1202 to control an exposure. A diaphragm 1203 controls the lightintensity as needed, and an image is formed on a solid-state imagesensing device 1204. A signal output from the solid-state image sensingdevice 1204 is processed by a captured-image signal processing circuit1205 and is converted by an A/D converter 1206 from an analog signal toa digital signal. The output digital signal is further subjected tocomputational operations by a signal processor 1207. The resultingdigital signal is stored in a memory 1210 and/or is transferred to anexternal apparatus via an external interface (I/F) 1213. A timinggenerator 1208 controls the solid-state image sensing device 1204, thesignal processing circuit 1205, the A/D converter 1206, and the signalprocessor 1207. An entire-control/operation section 1209 controls theentire system. The output digital signal is recorded as image data in arecording medium 1212 via a recording-medium control interface (I/F)1211, which is controlled by the entire-control/operation section 1209.

Not only can the present invention substantially increase thesensitivity of a CMOS solid-state image sensing device, but also allowsfor the configuration of a photodiode having an increased volume of ann-type layer compared to conventional configurations. This can thereforeincrease saturation charge in the image sensing device.

In addition, the doping concentration of the junction portion where thep-type surface layer 510 and the n-type epitaxial layer 503 of theburied photodiode are in contact with each other is lower than that ofthe conventional configurations. Thus, the present invention can reducethe rate of pixel defects, thereby improving the quality compared to theconventional examples.

Sixth Embodiment

A sixth embodiment will now be described with reference to FIG. 6. FIG.6 is a schematic sectional view of a solid-state image sensing deviceaccording to a sixth embodiment of the present invention. Since thewiring portions except the gate electrode are the same as those in thefifth embodiment, the common portions are not shown.

What is different from the fifth embodiment is that another n-type layer612 is provided adjacent to the surface of an n-type epitaxial layer 603in the photodiode structure. The n-type layer 612 is placed under ap-type surface layer 610 and also extends to a region under part of apolysilicon gate electrode 609. The doping concentration of the n-typelayer 612 is higher than that of the n-type epitaxial layer 603, and is,preferably, about 1E15 to 1E17 (/cm³). Since the n-type layer 612 is aregion where the potential for electrons is low, electrons gather in then-type layer 612 when charge is being stored. Thus, the structure ofthis embodiment is suitable for completely transferring electrons whenthe MOS transistor transfers charge. Complete transfer of the electronscan eliminate a fluctuation in the number of electrons remaining in thephotodiode, which makes it possible to configure an image sensing devicehaving low random noise.

The n-type layer 612 is formed by patterning and ion implantation in aprocess prior to the formation of the polysilicon gate electrode 609.Alternatively, after the polysilicon gate electrode 609 is formed, then-type layer 612 can be laid under the polysilicon gate electrode 609 byoblique ion implantation. Other portions in the sectional view of FIG. 6are the same as those in the fifth embodiment. P-type isolation layers604 a and 604 b and p well layers 605 a and 605 b, which are criticalportions in the present invention, can be formed using the same methodas for the fifth embodiment. Since other portions are analogous, thedescriptions thereof will be omitted.

This embodiment offers an advantage in that charge being stored iseasily transferred by the transfer MOS transistor, since a portionhaving a doping concentration higher than the other portions is providedadjacent to the surface of the cathode portion of the photodiode. Thisstructure facilitates complete transfer of photodiode charge, which canachieve a device having less random noise. This embodiment can also beapplied to the plane structure in FIG. 8, the circuit configuration inFIG. 11, and the block configuration in FIG. 12, which provides an imagesensing system that operates effectively.

Seventh Embodiment

A seventh embodiment will now be described with reference to FIG. 7.FIG. 7 is a schematic sectional view of a solid-state image sensingdevice according to a seventh embodiment of the present invention.

In this embodiment, in terms of dopant concentration, p-type isolationlayers 704 a and 704 b are not in complete contact with p well layers705 a and 705 b and a heavily-doped p-type buried layer 702. Even withthis structure, it can advantageously be used as long as these p-typeregions 704 a, 704 b, 705 a, 705 b, and 702 form sufficient potentialbarriers against the n-type region (an n-type epitaxial layer 703) ofthe photodiode. The potential barriers can be determined from the amountof charge desired to be stored in the photodiode. While the thickness ofthe n-type epitaxial layer in the fifth embodiment is 2 to 6 μm, thenumber of isolation layers can be increased in the depth direction inorder to form potential barriers with the p-type layers. Since otherportions in the sectional view of FIG. 7 are the same as those in thefifth embodiment, the description thereof will be omitted.

As illustrated in this embodiment, the present invention canadvantageously be used as long as sufficient potential barriers areformed even when the isolation layers are out of contact with the p welllayers and the p-type buried layer. That is, it is sufficient to arrangethe number and doping concentration of required isolation layersaccording to the essence of this embodiment, depending on the thicknessof the n-type epitaxial layer.

Eighth Embodiment

An eighth embodiment will now be described with reference to FIG. 9.FIG. 9 is a schematic sectional view of a solid-state image sensingdevice according to an eighth embodiment of the present invention. Thisembodiment illustrates a case in which STI (Shallow Trench Isolation) isemployed as an element isolation method. In this embodiment, in terms ofdopant concentration, p-type isolation layers 904 a and 904 b are not incomplete contact with p well layers 905 a and 905 b and a heavily-dopedp-type buried layer 902. This structure is the same as that of theseventh embodiment.

Referring to FIG. 9, p-type regions 906 a and 906 b are arranged underSTI oxide films 907 so that the interfaces between the STI oxide films907 and the silicon are not depleted. The p-type regions 906 a and 906 bare particularly significant in order to prevent the side surfaces ofthe STI oxide films 907 from being depleted. The p well layers 905 a and905 b are formed at the same depth as those of the seventh embodiment,and prevent charge from leaking into adjacent pixels. Further, the pwell layers 905 a and 905 b control the transfer path for the transferMOS transistor and also control a threshold thereof. The p well layer905 b, which is located immediately under the STI oxide film 907,provides a potential barrier immediately under the device-isolatingregion and also serves as a channel stopper under the STI oxide film907.

As in the other embodiments, the p-type isolation layers 904 a and 904 bprovide potential barriers between the p well regions 905 a and 905 band the heavily-doped p-type buried region 902.

Since other structures are the same as those of the sixth and seventhembodiments, the descriptions thereof will be omitted. Since STI is usedto isolate elements, this embodiment has additional advantages.Specifically, isolation between adjacent photodiodes is enhanced.Further, the shape of a resist is stabilized even when micro-processingis involved, since the surface is flat. In addition, since ionimplantation can be used to form the p-type isolation layers and the pwell layers after the formation of the STI oxide films, a micro-patterncan be formed such that the p-type isolation layers and the p welllayers do not expand due to heat diffusion during oxidation.Accordingly, this embodiment using STI is suitable for manufacture of animage sensing device having micro pixels.

Ninth Embodiment

A ninth embodiment will now be described with reference to FIG. 10. FIG.10 is a schematic sectional view of a solid-state image sensing deviceaccording to a ninth embodiment of the present invention. Thisembodiment illustrates a case in which DTI (Deep Trench Isolation) isemployed as an element isolation method.

In this embodiment, in terms of dopant concentration, a p-type isolationlayer 1004 and a p well layer 1005 are not in complete contact with aheavily-doped p-type buried layer 1002. This structure is the same asthose of the seventh and eighth embodiments. Trench-isolation oxidefilms 1007 extend to a depth of 3 μm in the silicon. P-type regions 1006a and 1006 b cover the interfaces between the trench-isolation oxidefilms 1007 and the silicon to prevent the interfaces thereof from beingdepleted. The p-type regions 1006 a and 1006 b prevent dark current frombeing generated and also form potential barriers between thetrench-isolation oxide films 1007 and the heavily-doped p-type buriedlayer 1002. Since other structures are the same as those of the sixth toeighth embodiments, the descriptions thereof will be omitted.

Since deep trench isolation is used to isolate elements, this embodimenthas additional advantages. Specifically, isolation between adjacentphotodiodes is further improved. Isolations regions that are narrow anddeep can be provided, thereby providing a structure suitable for micropixels.

As described above, according to the present invention, since thepotential barriers are provided in the pixel structure, signal charge isless likely to be absorbed by portions other than the photodiode. Thus,it is possible to achieve a solid-state image sensing device having highsensitivity. In addition, according to the present invention, it ispossible to provide a photodiode having an n-type layer large in volume,compared to the conventional configurations. Thus, it is possible toincrease saturation charge in the image sensing device.

Additionally, the doping concentration of the junction portion where then-type epitaxial layer and the p-type surface layer of the buriedphotodiode are in contact with each other is lower than the dopingconcentration of the conventional configurations. Thus, the presentinvention can reduce the rate of pixel defects, thereby improving thequality compared to the conventional examples.

Furthermore, a portion having a higher doping concentration than that ofthe other portions is provided adjacent to the cathode portion of thephotodiode to thereby facilitate complete transfer of photodiode charge.Thus, it is possible to achieve a solid-state image sensing devicehaving low random noise.

While the present invention has been described with reference to whatare presently considered to be the preferred embodiments, it is to beunderstood that the invention is not limited to the disclosedembodiments. On the contrary, the invention is intended to cover variousmodifications and equivalent arrangements included within the spirit andscope of the appended claims. The scope of the following claims is to beaccorded the broadest interpretation so as to encompass all suchmodifications and equivalent structures and functions.

1. A solid-state image sensing device having a plurality of pixels, each pixel comprising: a first semiconductor region of a first conductivity type; a second semiconductor region of a second conductivity type opposite to the first conductivity type, the second semiconductor region constituting a PN junction with the first semiconductor region and functioning as a part of a photodiode; and an element isolation region having an insulating film in a trench; wherein the first semiconductor region is disposed at a depth deeper than that of the second semiconductor region with respect to a light receiving surface; a third semiconductor region of the first conductivity type disposed on a side wall of the element isolation region; a fourth semiconductor region of the first conductivity type disposed under the element isolation region; a fifth semiconductor region of the second conductivity type having an impurity concentration higher than that of the second semiconductor region and disposed at a depth shallower than that of the second semiconductor region with respect to the light receiving surface, said fifth semiconductor region functioning as an accumulation part of the photodiode; a transistor of the second conductivity type in which the fifth semiconductor region functions as a source region or a drain region, wherein the fourth semiconductor region is disposed continuously to the first semiconductor region and electrically isolates an adjacent photodiode, wherein the fourth semiconductor region comprises a plurality of semiconductor regions stacked in the direction of depth; and a sixth semiconductor region of the first conductivity type disposed within the second semiconductor region directly under a gate electrode of the transistor in the direction of depth apart from a channel formed under the gate electrode, and wherein at least a portion of the fifth semiconductor region extends directly under a gate electrode of the transistor in the direction of depth of the device.
 2. A solid state image sensing device according to claim 1, wherein the sixth semiconductor region comprises a plurality of semiconductor regions stacked in the direction of depth.
 3. A camera system comprising the solid state image sensing device according to claim
 1. 4. A solid state image sensing device according to claim 1, wherein the fourth semiconductor region, the sixth semiconductor region and the first semiconductor region are formed continuously.
 5. A solid state image sensing device according to claim 1, wherein the third semiconductor region and the fourth semiconductor region are continuously formed.
 6. A solid state image sensing device having a plurality of pixels, each pixel comprising: a first semiconductor region of a first conductivity type; a second semiconductor region of a second conductivity type opposite to the first conductivity type, the second semiconductor region constituting a PN junction with the first semiconductor region and functioning as a part of a photodiode; an element isolation region having Shallow Trench Isolation forming an insulating film, wherein the first semiconductor region is disposed at a depth deeper than that of the second semiconductor region with respect to a light receiving surface; a third semiconductor region of the first conductivity type disposed on a side wall of the element isolation region; a fourth semiconductor region of the first conductivity type disposed under the element isolation region; a fifth semiconductor region of the second conductivity type having an impurity concentration higher than that of the second semiconductor region and disposed at a depth shallower than that of the second semiconductor region with respect to the light receiving surface, said fifth semiconductor region functioning as an accumulation part of the photodiode; a transistor of the second conductivity type in which the fifth semiconductor region functions as a source region or a drain region, and wherein the fourth semiconductor region is disposed continuously to the first semiconductor region and electrically isolates an adjacent photodiode, wherein the fourth semiconductor region comprises a plurality of semiconductor regions stacked in the direction of depth; and a sixth semiconductor region of the first conductivity type disposed within the second semiconductor region directly under a gate electrode of the transistor in the direction of depth apart from a channel formed under the gate electrode, and wherein at least a portion of the fifth semiconductor region extends directly under a gate electrode of the transistor in the direction of depth of the device wherein the fourth semiconductor region is a potential barrier.
 7. A solid state image sensing device according to claim 6, wherein the sixth semiconductor region comprises a plurality of semiconductor regions stacked in the direction of depth.
 8. A camera system comprising the solid state image sensing device according to claim
 6. 9. A solid state image sensing device according to claim 6, wherein the fourth semiconductor region, the sixth semiconductor region and the first semiconductor region are formed continuously.
 10. A solid state image sensing device according to claim 6, wherein the third semiconductor region and the fourth semiconductor region are continuously formed. 